Persistent phosphorescence was discovered in the 11th century in China and Japan and in the 16th century in Europe (Shionoya, S. (1998) in Phosphor Handbook, Shionoya, S; and Yen, W. M.; (eds.), CRC Press, Inc., Boca Raton, New York, p. 3). The phenomenon involves two kinds of active centers, emitters and traps. Emitters are centers capable of emitting radiation after excitation of the center. Traps do not emit radiation, but store excitation energy and release it gradually to the emitter. Emitter centers can be created through the addition of activators, i.e., small amounts of intentionally added impurity atoms or ions, to the host matrix. Co-activators are additional, intentionally-added impurity ions which may affect (improve or modify) the emission of an activator. For example, a co-activator can be added to form trapping centers that can increase the persistence time of the phosphor.
Thermoluminescence and photostimulable phosphorescence are physically governed by mechanisms similar to persistent phosphorescence (Chen, R. and McKeever, S. W. S., (1997), Theory of Thermoluminescence and Related Phenomena, World Scientific, Singapore; Sonoda, M. et al. (1983) Radiology 148:833). The main difference between these three phenomena appears to be the depth of traps. When a phosphor possesses centers with certain trapping depths that can be effectively thermally activated at room temperature, it will show persistent phosphorescence. Deeper trapping centers can be activated by heating or photostimulation. Measurement of the thermoluminescence of phosphor samples can be used to characterize the traps present in the phosphor. Because of mechanistic similarities, a study of phosphorescence dynamics allows the characterization of these three important luminescence processes. Single crystal phosphors facilitate studies of phosphorescence dynamics.
Persistence of phosphorescence is measured herein as persistence time, which is the time, after discontinuing irradiation, that it takes for phosphorescence of a sample to decrease to the threshold of eye sensitivity. This threshold is the signal level of emission intensity that an unaided eye can perceive in the dark. Persistence times are assessed by following phosphorescence intensity as a function of time. Measurement comparisons of persistence times must be performed under identical conditions using the same detection systems.